Propylene and butadiene are important chemical raw materials, which are usually obtained from naphtha cracking and steam cracking. The main sources of propylene are co-production of ethylene with propylene and by-product of refinery. The main source of butadiene is the further processing of C4 by-product produced in ethylene cracking process. In recent years, the technologies of methanol to olefin (MTO), methanol to propylene (MTP), ethane dehydrogenation to ethylene and propane dehydrogenation to propylene have been rapidly developed. There is an obvious tendency of raw material lightening in global olefin production, which will lead to the shortage of C4 resources. Therefore, it is necessary to develop a process that can produce propylene and C4 olefins with a high selectivity to meet market demand.
The fixed-bed methanol-to-olefin technology (WO2004/018089) was developed by LURGI AG in Germany. The technology utilized a ZSM-5 molecular sieve catalyst from Sud-Chemie AG to carry out methanol-to-olefin reaction in a fixed-bed reactor. The selectivity of propylene was close to 70%, and the by-products were ethylene, liquefied petroleum gas and gasoline.
The DMTO technology developed by Dalian Institute of Chemical Physics used a SAPO molecular sieve as catalyst, a dense-phase circulating fluidized-bed reactor and a methanol aqueous solution as raw material. The yield of ethylene and propylene in the product was about 80%, and more than 10% of C4 hydrocarbons were yielded as by-products.
Patent CN104098429A discloses a method of preparing propylene and C4 hydrocarbons from methanol in a circulating fluidized-bed using a ZSM-5 catalyst. The process features are that the raw material methanol and most of C1, C2 and C5 hydrocarbons in the product are entered into the circulating fluidized-bed reactor together, and propylene, C4 hydrocarbons, hydrocarbons of C6 and above and by-products are recovered as final products.
Patent CN101177374B discloses a method for preparing olefins from methanol or dimethyl ether. The method includes the conversion of methanol or dimethyl ether, the alkylation of ethylene and methanol, and the catalytic cracking of components heavier than C4. Catalyst 1 is used for the methanol or dimethyl ether conversion and the ethylene and methanol alkylation in one reactor, and catalyst 2 is used for the catalytic cracking of components heavier than C4 in another reactor.
The methods disclosed in patents CN104098429A and CN101177374B share a common feature, that is, the selectivity of target products (propylene and C4) is increased through the recycling of light fractions (hydrocarbons with a carbon number of no more than 2). The alkylation of ethylene with methanol is the main reaction in the recycling reaction of the light fractions mentioned above.
The acidic molecular sieve catalysts can be used in both MTO reaction and alkylation of olefins. However, the rate of the MTO reaction is much higher than that of the alkylation of olefins. We have found that a fresh SAPO catalyst has a high activity, which is more beneficial to the alkylation of olefins. After a carbon deposition of catalyst, the reaction rate of alkylation of olefins will decrease rapidly.
Methanol is not only the raw material for the alkylation of olefins, but also the raw material for the MTO reaction. Therefore, the alkylation of olefins is necessarily accompanied by the MTO reaction. The MTO reaction will lead to a carbon deposition and lower activity of catalyst, which will hence inhibit the alkylation of olefins. An increase in the alkylation rate of olefins can reduce the content of light fractions in the product gas, and thus the unit volume production capacity of the reactor can be increased.
The methods disclosed in patents CN104098429A and CN101177374B do not refer to the reactor structure, nor do they clarify the flow modes of catalyst and raw material and the raw material distribution in the reactor. The method disclosed in patent CN101177374B uses a SAPO catalyst. The examples show that the mass ratio of methanol to light fractions is 1:10-20. Thus, it can be seen that the content of light fractions is very high and the unit volume production capacity of reactor is very low. A ZSM-5 catalyst is used in the method disclosed in patent CN104098429A. The content of hydrocarbons of C6 and above in the product is relatively high. The content of light fractions in the product gas is not disclosed in this method.
From the above analysis, it can be seen that the main reactions for the preparation of propylene and C4 hydrocarbons from methanol are the MTO reaction and the alkylation of olefins. Therefore, the key to improve the selectivity of propylene and C4 hydrocarbons lies in a catalyst design and a reactor design. Avoiding the inhibition of the MTO reaction to the alkylation of olefins through an optimization in the reactor design is one of the important methods to improve the economics of methanol to propylene and C4 hydrocarbons.